The present invention relates to a flexible electrical heating or temperature measuring element made of electrically insulated conductive lines for feeding power to one or more heating or measuring conductors, as the case may be, and more particularly the invention relates to elements of the type above wherein the power feeding conductors intermittently connected to the heating or measuring conductor, along the length of the element and wherein zones are defined in between sequential connection points for purposes of applying or sensing heat.
Flexible heating elements of the type to which the invention pertains are known, for example, under the trade name "Econotrace". These elements provide for heating along their extension and include feeder conductors made of copper with an insulation of polytetrafluoroethylene-perfluoropropylene (FEP). The same insulation is provided around both of the feeder conductors constituting the so-called internal insulation, and the heating conductor proper is round on top thereof. This heating conductor is comprised, for example, of a chromium-nickel alloy. The aforementioned internal insulation as well as the jacket on the individual feeder conductors or lines is removed along the outer edges and in certain spaced apart locations so that the adjacent heating conductor is electrically connected with the conductors of the feeder line in these locations. This arrangement is basically satisfactory. However, it was found that due to the required flexibility of the cable or element as a whole, it may occur that the loops of the heating conductors shift and particularly, they may shift away from the location where the insulation had been removed from the feeder conductors. Therefore, satisfactory and consistent electrical contact is not guaranteed. The same problem has been observed if upon heating the various elements and materials expand, thereby for example, loosening the loops of the heating conductor from one or the other locations.
The aforementioned system as well as the so-called self-regulating heating strip as disclosed, for example, in U.S. Pat. No. 4,194,536, are disadvantaged further by the fact that for monitoring or control of the heating current one needs additional cable or conductors. Even more important, however, is the fact that the power consumption and dissipation in the known heating element, including those of the self-regulating variety, is subject to limitations. This means that, for example, the heating or other thermal control, for example, of large tubes conducting fluid which has to be kept at a certain temperature during conduction requires a large amount of individual heating elements each being provided with connection pieces and feed points. It has to be considered here that particularly in the field of so-called tube bundle cables heating, or other temperature controls, requires frequently that branches in the conduit system have to be accommodated, particularly they have to be likewise heated or temperature controlled. This requires additional feed points for heating and there may be a problem with regard to mounting and connecting space, particularly in the branch point. In this regard it has to be considered that the connection branches and other joints usually require particular protection against ingress of moisture, and sometimes even explosion proofing is required.
The same is essentially true with regard to a different kind of known heating cable traded under the designation "W082/01112" wherein the heating conductor runs in between the parallelly arranged feeding lines. The electrically conductive connection between the feeder lines and the heating conductor is provided intermittently under utilization of clamping type contact elements. The heating conductor may be comprised, in this case, of a helix that is round on top of a core and it is a core which extends in parallel to the feeder lines.